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title - Do not simultaneously open a file multiple times

The behavior when simultaneously opening a file multiple times is implementation defined. On some platforms, this is not even allowed. On others, it might result in race conditions.

Non-Compliant Coding Example

The following non-compliant code opens the file log twice concurrently.

void do_stuff(void) {
  FILE* logfile = fopen("log", "a");
  
  /* openCheck logfor fileerrors, write logs pertaining to do_stuff, call log_function, open log file, write */

Compliant Solution

 etc. */
}

int main() {
  FILE* logfile = fopen("log", "a");

  /* Check for errors, write logs pertaining to main, etc. */

  do_stuff()

  /* ... */
}

Compliant Solution

In this compliant solution, a reference to the file pointer is passed around so that the file does not have to be opened twice separately.

void do_stuff(FILE **file) {
  FILE* logfile = *file;
  
  /* pass around FILE **file_ptr */ Check for errors, write logs pertaining to do_stuff, etc. */
}

int main() {
  FILE* logfile = fopen("log", "a");

  /* Check for errors, write logs pertaining to main, etc. */

  do_stuff(&logfile)

  /* ... */
}

Risk Assessment

Simultaneously opening a file multiple times could result in abnormal program termination or a data integrity violation.

References

\[[ISO/IEC 9899-1999:TC2|AA. C References#ISO/IEC 9899-1999TC2]\] Section 7.19.3, "Files"